1. Field of the Invention
The invention relates to a method for controlling a piezoelectric drive and a piezoelectric drive suited for the implementation of the method.
2. Description of the Prior Art
Piezoelectric drives can find an application in various fields. An application are proportioning devices, particularly for small volumes proportioned in the nanolitre to millilitre ranges.
WO 99/10099 discloses various microproportioning systems having an open-jet proportioner and/or a micro-diaphragm pump. Microdiaphragm pumps and open-jet proportioners have a displacement chamber and a deformable displacement wall or diaphragm associated therewith which can be driven by a piezoelectric actuator. The displacement chamber is connected to a reservoir for the supply of liquid and feeds a proportioning port which can be formed as a nozzle. In a microdiaphragm pump, the liquid flows off or drops out of a proportioning port. In an open jet proportioner, in contrast, the liquid is expelled from the proportioning port in an open jet. From this document, it has been known already to install a fluid module from a reservoir and a displacement chamber in a reception duct of a proportioning device which has an actuator associated with the reception duct which acts on the diaphragm of the module.
WO 99/37400 has made known another microproportioning device which has a proportioning module which is exchangeably arranged in a casing. The proportioning module comprises a proportioning chip with a displacer diaphragm and a pressurized chamber which is connected to a nozzle and a medium reservoir. Also here, the actuator of the microproportioning device and the displacer of the proportioning module are not fixedly connected to each other. Rather, the actuator is merely brought into contact with the displacer. The actuator is a piezoelectric stack actuator (xe2x80x9cpiezostackxe2x80x9d) the force of which is transmitted to a pressure point of the proportioning chamber via a rotatably supported lever actuator. The proportioning volume is dependent on the voltage applied to the piezostack. In another embodiment, a control signal U (t) of a low edge steepness is provided to the actuating device at an interval called a suction phase, which causes a slow actuation of the diaphragm out of the initial position. In a proportioning phase, the control signal is brought down to a low level at a very short interval to expel the liquid through the nozzle in an open jet.
The voltage control which has been known before has the disadvantage that the accuracy in deflecting the piezostack does not always satisfy the requirements.
Accordingly, it is the object of the invention to provide a method for controlling a piezoelectric drive at an improved accuracy. In addition, the aim is to provide a suitable piezoelectric drive for implementing the method.
The object is attained by a method for controlling a piezoelectric drive in which:
a required voltage to be applied to a piezoactuator for a desired deflection of the piezoactuator is determined by means of the deflection characteristic curve of the piezoactuator,
the piezoactuator is supplied with a current by means of a current source,
the voltage is measured on the piezoactuator while the piezoactuator is being supplied with the current from the current source,
the voltage measured is compared to the voltage required, and
the supply of the piezoactuator with the current from the current source is turned off once the required voltage is reached.
A piezoelectric drive which is suitable for implementing the method has a piezoactuator,
a device for determining a required voltage to be applied to the piezoactuator for a desired deflection of the piezoactuator,
a current source connected to the piezoactuator for supplying the piezoactuator with a current,
a voltage measuring device connected to the piezoactuator for measuring the voltage on the piezoactuator,
a device connected to the voltage measuring device and the device for determining a required voltage to compare the voltage measured and the voltage required, and
an electric control device connected to the device for comparing and the current source to turn off the source of the piezoactuator with the current of the current source voltage once the required voltage is reached.
Analyses made on the voltage control, which has been known before, in conjunction with the present invention have revealed that a predetermined voltage will adjust itself only gradually on the piezostack because the latter electrically corresponds to a capacitor so that the voltage will rise according to an exponential function. However, since the deflection of the piezostack is proportional to the voltage the deflection desired will be reached only after a certain time lag. Moreover, the exponential variation of deflection is accompanied by a steadily varying dynamics. The additional undesirable side effect which presents itself in voltage control is a dielectric hysteresis, which makes it very difficult to ascertain the relationship between the voltage applied, dynamics, and deflection. What adds to such effect in the aforementioned embodiment that uses the control signal exhibiting control edges is the change in time of the triggering signal
Instead of working with a voltage source, the inventive method and the piezoelectric drive use a current source. The current source features a large internal resistance so as to supply a substantially constant current. The internal resistance of an ideal current source tends to approach infinity. Charging or discharging the piezoactuator with an impressed constant current achieves a linear rise or drop of the voltage on the piezoactuator and, as a result, the gradient or dynamics of this rise or drop is dependent on the driving current strength. Studies made in conjunction with the invention have also shown that this way of triggering avoids or minimizes a hysteresis of the path.
The easiest way of forcing the piezoactuator to effect a defined deflection by means of current control would consist in using a timed current source. The accuracy required for the current source and its timing could be ensured at a relatively low expenditure. However, the voltage arising during the time of current application and, hence, the deflection of the piezoactuator are also a function of piezostack capacitance. More detailed studies have proved that it does not react like an ideal capacitor, but that capacitance varies with the deflection of the piezoactuator with no linear correlation existing between the capacitance and deflection. Furthermore, dependencies of capacitance on the temperature, ageing, and the mechanical bias on the piezoactuator would result in inaccuracies relating to voltage and, hence, to deflection.
Therefore, the invention uses a current source feeding the piezoactuator and measures the voltage which actually is applied to the piezoactuator while the current is being fed. The voltage measured is compared to the voltage required to achieve the desired deflection which can be determined by means of the deflection characteristic curve of the piezoactuator, i.e. the dependence of the piezoactuator deflection on the voltage applied. Once the voltage measured matches the voltage required the piezoactuator is disconnected from the supply and, hence, the desired deflection is exactly achieved. This is the way the piezoactuator may both be charged and discharged to expand or contract the piezoactuator, i.e. to deflect it in various directions.
In this mode of operation, a non-constant capacitance of the piezoactuator still has only a small influence on the dynamics of deflection, but no longer on deflection itself. The accuracy of deflection is primarily governed by the accuracy at which the voltage is measured on the piezoactuator. Such accuracy may be ensured by a calibrated voltage measuring device. In contrast, the current source may basically be operated in an unbalanced state, nor needs the capacitance of the piezoactuator be taken into account in any way. There is no longer any dependence on the capacitance of the piezoactuator that has a tolerance and is not constant in a dynamic mode of operation, nor is there still any dependence on the mechanical bias, state of ageing, and temperature. This significantly improves the accuracy of deflection while maintaining the accuracy of deflection, and reduces the expenditure in manufacture and service. This is an advantage specifically for applications in proportioning devices.
The invention may be generally employed in any piezoactuators. It is employed with great advantage in piezostacks because the above described capacitance influences are particularly pronounced in piezostacks.
The aspects of the method are as follows:
The dynamics of deflection depends on the current with which the piezoactuator is fed. According to an aspect, the current source is an adjustable current source so that adjusting the current will influence the dynamics of deflection. Adjusting the current source allows to preset a certain dynamics for the entire deflection. The dynamics of deflection may further be influenced by making adjustments during deflection.
According to another aspect, the piezoactuator is controlled in such a way that expansion and contraction are effected at different dynamics. This can be utilized, in particular, for the deformation of the displacement wall of displacement chambers in proportioning devices.
According to an aspect, the piezoactuator acts on a deformable displacement wall of a displacement chamber for proportioning fluids and the deflection of the piezoactuator for a desired proportioning volume of the fluid is determined by means of a proportioning characteristic curve of the displacement chamber and the piezoactuator is controlled accordingly. Thus, the displacement chamber forms part of a piezoelectric drive which causes fluids to be proportioned. The proportioning characteristic curve is the dependence of the displaced fluid volume on the deformation of the displacement wall that can be determined for the displacement chamber.
According to an aspect, the desired proportioning volume is received in and/or expelled from the displacement chamber by means of a single deflection. This is employed for relatively small, lower proportioning volumes which fall below the displacement volume of the displacement chamber. According to another aspect, the desired proportioning volume is received in and/or expelled from the displacement chamber by means of several deflections. This particularly applies to relatively large proportioning volumes which exceed the displacement volume of the displacement chamber.
If the voltage required is not achieved by the voltage measured on the piezoactuator the desired deflection is not achieved either. Therefore, according to an aspect, an error message is generated if the required voltage is not achieved.
The aspects of the piezoelectric drive are as follows:
According to an aspect, the piezoactuator and the electric control device have connected thereto a current source for charging and a further current source for discharging in order to control an expansion and contraction of the piezoactuator.
The voltage measuring device and the device for comparing may be different devices. According to an aspect, the device for determining the required voltage is connected to a differential voltage measuring device and the differential voltage measuring device is connected to the control device in order to switch the current off if the difference voltage from the voltage and the required voltage reaches the zero value. The voltage measuring device and the device for comparing coincide in the differential voltage measuring device.
According to an aspect, the current source and/or the other current source are connected to an electric voltage supply which comprises at least one battery and/or at least one accumulator and/or a power supply unit. According to a further aspect, a high voltage which is required to operate the piezoactuator is produced by a voltage converter of the electric voltage supply.
The supply of electric current into the piezoactuator may be switched on and off in different ways. According to an aspect, an electric switching mechanism is disposed between the electric voltage supply and the current source and/or the further current source and is connected to the electric control device in order to switch on and off the voltage supply of the current source and/or the further current source.
According to an aspect, the electric control device comprises a microcomputer, i.e. it works under the control by a software. According to another aspect, the electric control device is a hardware control unit.
According to an aspect, the piezoelectric drive comprises a displacement wall of a displacement chamber for proportioning liquids in order to expel a fluid from/draw a fluid into the displacement chamber by deforming the displacement wall. The displacement chamber will then form part of the piezoelectric drive which causes fluids to be proportioned. The displacement chamber and further components of a microproportioning device may be designed specifically as described in WO 99/100999 and WO99/37400. The relevant indications in the aforementioned documents are incorporated in this application by reference thereto.
According to a further aspect, devices are provided for inputting a deflection of the piezoactuator and/or a proportioning volume.
Apart from using the method and piezoelectric drive in the proportioning technology, a great variety of further applications thereof are possible in which it matters that the piezoactuator be deflected precisely, particularly in microactuator drives.